Thermal energy storage technology is important because it saves energy, provides economic benefits, and permits the convenient use of periodic energy sources, such as solar energy. Certain thermal energy storage systems using the specific heat of water, rocks, and ceramics are already commercially available. Systems that contain phase change materials are being developed because a high energy storage density is associated with the change of phase. Long term thermal energy storage can be achieved by means of heats of solution, hydration, and reaction of certain chemicals. All of these thermal energy storage materials have one or more of the following technical difficulties that must be overcome: agglomeration, component separation, supercooling, large volume in comparison with thermal capacity, low thermal conductivity, expensive heat exchanger requirements, corrosion of container walls, incompatibility with system components, and limited surface area.
One attempted method to circumvent these problems was to form construction composites of the thermal energy storage materials with concrete or plastic, but new problems of seepage, component incompatibility, and reduced thermal conductivity occurred. Brookhaven National Laboratory reports BNL 50827 (August 1977-February 1978) and BNL 50896 (March 1978-May 1978), for example, discuss the problems with, and failures experienced in, attempting to incorporate phase change materials, both the inorganic salt hydrate (e.g., CaCl.sub.2.6H.sub.2 O, Na.sub.2 SO.sub.4.10H.sub.2 O) and organic (e.g., fatty acids, polyethylene glycol) types, into ordinary concrete, polymer-impregnated concrete and polymer concrete. The project met with limited success in its attempts to introduce phase change materials into concrete, and that little success was primarily achieved using foamed glass beads impregnated with CaCl.sub.2.6H.sub.2 O. The incorporation of thermal energy storage materials into plastics is disclosed in U.S. Pat. No. 4,003,426. The thermal energy storage material is dispersed into an uncured polymeric resinous matrix which is then cross-linked. As stated in the patent, this method is useful only with thermal energy storage materials which will form stable dispersions in the uncured polymer and requires the envelopment of the cured structure in a gas or vapor barrier material for best results.
Another attempted method for overcoming the problems was macro-encapsulation. In this case, thermal energy storage materials about 1 inch diameter or larger are jacketed with multilayer flexible plastic/metal film composites, steel cans, or polyolefin bottles. This approach may be useful for certain applications but commercialization has been hampered by poor thermal conductivity, deformation of the packages, and degradation of the encapsulating materials by chemical attack and mechanical stresses.
Still another approach is microencapsulation. Research on thermal energy storage systems containing microencapsulated wax demonstrated the need for encapsulated thermal energy storage materials and the technical feasibility of their use. The results proved that microencapsulated thermal energy storage material can be packed into a bed through which heat exchange fluid passes. In this manner the heat exchange fluid directly contacts the heat storing material. The results also included improved thermal conductivity, reduced complexity in the heat exchanger, reduced thermal energy storage material separation problems, and reduced equipment costs. However, overall system costs were found to be too high with first generation microencapsulated thermal energy storage material due to the high processing costs of the encapsulating procedure. This work is described in report NSF/RANN/SE/AER 74-09186, dated November 1975. Subsequent work has not significantly altered these conclusions.
One of the principal goals of microencapsulation was to reduce thermal conductivity path lengths in heat transfer systems by reducing the size of the discrete thermal energy storage material particles employed. Capsules were made by coating paraffin particles in the 8-2000 micron (0.003"-0.08") range with wall-forming materials, such as gelatin (which was generally unsatisfactory), modified nylon and urea-formaldehyde resin. It was observed that the 50 micron nylon-coated particles exhibited no leakage after 300 thermal cycles, while the 1000-2000 micron samples exhibited some leakage after 150 cycles. These results notwithstanding, the optimum cost that could be achieved using the microencapsulation techniques of the prior art was three to four times the maximum break-even figure for commercial application. Consequently, this approach was dropped until such a time that a less costly encapsulating procedure, particularly for particles in the 8-2000 micron range, was devised or until a method was discovered to provide encapsulation of water-soluble salts.
Phase change materials (PCMs) for thermal energy storage applications offer advantages over materials such as water or rocks, whose thermal storage capacity is based exclusively on sensible heat. These advantages include higher energy density, less temperature variation, and, as a consequence, higher heat collection efficiency. Nevertheless, the PCMs have their drawbacks. Compared with water or rocks they are generally higher in cost, lower in heat transfer rates, and more corrosive. In addition, many PCMs can undergo phase separation. Widespread application of PCMs in heat storage systems will depend on cost effective packaging materials and configurations for the PCMs, and appropriate heat exchange systems.
Although each of the earlier-discussed approaches to energy storage offers a partial answer to heat storage problems, a versatile solution that combines flexibility in material selection, full utilization of latent heat, high production efficiency, and long term reliability of the heat storage system remained to be invented. The product of this invention and the process for preparing the product provided such a solution.
Another serious problem must be overcome when phase-change materials are placed within rigid materials of construction, such as concrete, is the problem of expansion of the PCMs upon phase-changes, resulting in fractures and serious damage to the construction material. The product and process of this invention overcomes this otherwise serious problem.
This invention provides a process for solid state encapsulation of both water soluble and insoluble PCMs employing common, commercially available raw materials and well-known equipment. One aspect of the process involves pressing PCMs into aspirin-size tablets which are then coated with organic resin formulations. The size of the resulting capsules lies between that of plastic spheres and microcapsules. The capsules are small enough to provide efficient heat transfer and to prevent possible phase separation, yet are large enough for low-cost mass production. Because the capsules are designed to be used in large quantity for long term thermal cycling applications, the durability, flexibility, and cost of the capsule wall materials are important parameters.